Labyrinth seals are primarily used to reduce or control the internal leakage of fluid within such systems as gas and steam turbines, compressors, and pumps, where fluid flow between two relatively moving members generally occurs. More particularly, labyrinth seals are often used in sealing an element such as a rotary shaft to inhibit undesirable fluid flow past the shaft. When incorporated in a pump, the labyrinth seal is relied upon to inhibit leakage along the shaft of the fluid being pumped by the pump.
A labyrinth seal is generally characterized by a series of cavities or grooves formed along the adjacent surfaces of two relatively movable members, where these members defined a partial barrier between areas of high and low pressure. At successive stations or steps along the seal, the adjacent surfaces of these rotatable members are situated in close juxtaposition to each other such as to define annular slit-like orifices. Further in this type of seal design, a series of cavities or chambers are formed between these stations in order to retard fluid flow through the seal.
In operation, the previously described labyrinth design forms a seal between the rotatable members by forcing high velocity fluid to navigate the irregularly spaced adjacent surfaces formed between these relatively movable members, said fluid sequentially passing through the slit-like orifices to enter the enlarged cavities where the velocity of the fluid is largely dissipated in turbulence. In this, the basic concept of any labyrinth seal design is to create a highly frictional flow passage. Such a flow path will convert pressure energy into velocity energy, a large portion of which will be dissipated into heat via turbulent action.
One source of turbulence is created as a result of the wall shear friction between the high velocity fluid and the irregularly spaced adjacent surfaces of the seal. A second and more important source of turbulence results from the intense free shear layer friction between the high velocity leakage jet discharging from an orifice and the relatively slow moving fluid in the large cavity immediately downstream. As a result of the combination of these and other friction components, pressure energy is substantially reduce downstream of each orifice of a multi-cavity seal. The substantially reduced pressure in a given cavity formed downstream of a particular orifice results in smaller pressure changes occurring across the orifices further downstream, the ultimate effect resulting in reduced leakage across the seal as a whole.
A variety of labyrinth seal designs have evolved in the art to take advantage of the principles of dissipative fluid flow. One early design is seen in U.S. Pat. No. 1,020,699--Kieser. In this design, a centrifugal pump is provided with a stepped and grooved sealing surface such that the kinetic energy of the fluid flow across the sealing surface is somewhat dissipated through designed turbulence.
In another such design, U.S. Pat. No. 1,482,031--Parsons, a labyrinth seal is characterized by a radially stepped surface provided along the rotor, the stator being provided with a corresponding set of barrier members or collars disposed in close relationship thereto. In this fashion, high pressure fluid moving across the sealing surface will encounter interference; thus, minimizing leakage. In yet another design, U.S. Pat. No. 3,940,153--Stocker, the labyrinth seal is characterized by a succession of annular orifices or clearances between sealing teeth or knives on one member, and generally cylindrical surfaces or lands on the other. In combination, the sealing system defines a doubly recurved flow path from each orifice to the orifice next downstream.
The prior art systems, as exemplified above, employ the use of sharp turns of the through-flow fluid to provide additional fluid resistance. The through-flow fluid is forced to "zig-zag" or "serpentine" through the seal. The turning of the through-flow fluid in the prior art is achieved through the use of wall positioning the wall curvature. Not fully appreciating the kinetics involved in the turbulence generation/dissipation process in a sealing system, the prior art configurations were designed not from precise quantitative data, but from intuition and expectation. The concern of the prior art has been to increase the wall shear friction through the use of a long and tortuous flow path between each pair of annular orifices. By focusing on the use of wall shear stress, the prior art has neglected the turbulence generating potential of a free (i.e. away from wall) shear layer.
This reliance on wall shearing to turn the through-flow fluid has caused prior art devices to be characterized by a variety of "knives" and "lands." These features have been combined either acutely or irregularly to increase the wall shear friction between the fluid and the sealing surfaces of the seal. This has resulted in the sealing surfaces of prior art seals having an intricate and complex surface.
The prior art has continued to employ wall shear friction to turn the through-flow fluid. More particularly, prior art devices have relied upon wall positioning and wall curvature to turn the fluid and have failed to utilize the potential of turning available in configurations capable of producing free shear layers.
This failure is particularly evident when said prior art seals have been adapted for operation in intense operating environments, such as in military or space applications. To maintain an adequate seal in such harsh environment, the trend in the art has been toward an increase in the number of sealing cavities or chambers, as well as an increase in the complexity of the relative geometry of the interfacing sealing surfaces. The art has also been inclined toward the use of alreadable or honeycomb matrix materials attached to the stationary or the rotating member of the seal. In yet a further effort to achieve more effective sealing, the art has increasingly restricted the relative clearance between the moving seal members by diverting a portion of the fluid through a system of intricately machined slots.
As a result of these shortcomings in the art as thus described, many contemporary labyrinth seals have been quite expensive to produce due to the intricate machining required to finish each sealing surface. Such complex sealing surfaces have also been somewhat fragile and thus prone to failure in rigorous applications due to the inherent structural weakness of the thinly structured sealing teeth or buttresses. Further, the increasingly small, high or light tolerances of such seals between the movable members are often impractical where the rotary drive shaft is prone to move off center or wobble. Most important of these deficiencies, prior art sealing systems have generally failed to achieve optimum sealing efficiency in intense operating environments.